There is security software like Avast, Kaspersky, BitDefender and ESET that intercept secure connection certificates and send their own. https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message https://support.mozilla.org/en-US/kb/connection-untrusted-error-message [https://support.mozilla.org/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors?redirectlocale=en-US&redirectslug=Error+loading+web+sites Websites don't load - troubleshoot and fix error messages] http://kb.mozillazine.org/Error_loading_websites [https://support.mozilla.org/en-US/kb/what-does-your-connection-is-not-secure-mean What do the security warning codes mean] '''SSL_ERROR_NO_CYPHER_OVERLAP''' Your server apparently doesn't offer any cipher suites necessary to establish a secure https connection that are supported in Firefox. You can check what kind of cipher suites Firefox can make use of by visiting; https://www.ssllabs.com/ssltest/viewMyClient.html Firefox will just show the SSL_ERROR_NO_CYPHER_OVERLAP error when encountered any site using the RC4 cipher.

Do not understand. My neighbour uses Plusnet and he can load the sites without problems.

sometimes internet can be a traffic jam. iM not you to to the but reinstall your program and in in to cold reboot and last but not least window 10 has a recover tool built in in last option

Not using Win 10, I am on a MAC. Same problem using Firefox and Safari.